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Author Topic: Why does water bounce?  (Read 4091 times)

Offline DoctorBeaver

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Why does water bounce?
« on: 10/10/2007 21:48:44 »
I'm not sure if this is a chemistry or physics question so I'll try it here & see what happens.

When I drop a stone into water, the stone goes through the surface of the water. OK, there may be a few splashes due to the water "rebounding" (I don't know the technical term), but the stone doesn't bounce.

However, when I pour water over a stone, it doesn't just flow over the stone, it bounces & splashes all over the place.

If I hit a piece of wood with an axe, the blade will sink into the wood. If I hit the blade of the axe with a piece of wood, the blade will still sink into the wood. Whichever way I cause the contact between the wood & the axe blade, the outcome is the same. So why is the outcome apparently different when dropping a stone into water or pouring water over a stone?


 

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Why does water bounce?
« Reply #1 on: 11/10/2007 00:02:41 »
I'm not sure if this is a chemistry or physics question so I'll try it here & see what happens.

It is physics, and I will move it there shortly.

When I drop a stone into water, the stone goes through the surface of the water. OK, there may be a few splashes due to the water "rebounding" (I don't know the technical term), but the stone doesn't bounce.

However, when I pour water over a stone, it doesn't just flow over the stone, it bounces & splashes all over the place.


So why is the outcome apparently different when dropping a stone into water or pouring water over a stone?

But how is it different - apart from in magnitude - in both cases the water splashes.

OK, the differences in magnitude are twofold in cause:

Firstly, they are different amounts of water (you hit an axe against a small piece of wood it will cut right through, and the tow pieces of wood, if not held, will go flying apart - but if you hit the axe into a large piece of wood, it mat simply embed itself without fully cleaving it apart, or it may just cleave it, but the two heavy bits will just fall apart rather than fly apart).

Secondly, the water travelling down as it hits the stone will have plenty of space and energy to keep travelling down after it hit the stone.  Water thrown up from the stone will have to work against gravity, and will quickly fall back down, and when it does fall down it will quickly hit the main body of water (that was missing when the water was projected downwards), thus giving not a lot of space and time to move away from the point of impact.

Other differences are that the water poured down on the stone will already be turbulent, and so will easily break up, whereas a solid body of water at rest is not turbulent, and so may form larger droplets that will not travel as far.

Lastly, a difference between your axe and wood, and your water and stone, is that the stone is a blunt surface, whereas the axe is sharp.  If you pour water onto a sharp edge it will behave differently than if you splash it onto a blunt (flat) surface).
 

Offline eric l

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Why does water bounce?
« Reply #2 on: 11/10/2007 13:35:32 »
 

Offline DoctorBeaver

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Why does water bounce?
« Reply #3 on: 11/10/2007 16:45:03 »
Thank you, Eric.
 

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Why does water bounce?
« Reply #3 on: 11/10/2007 16:45:03 »

 

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